1. Field of the Invention
The invention relates to a heat-sensitive lithographic printing plate used in the field of offset printing and, more particularly, to a negative-working heat-sensitive lithographic printing plate for computer-to-plate (CTP), capable of directly making plate based on a digital signal from a computer.
Priority is claimed on Japanese Patent Application No. 2002-270063, filed Sep. 17, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
Presensitized plates (PS plates) have been used as lithographic printing plate in the offset printing field for a long time. A PS plate comprises a substrate having the surface subjected to a hydrophilization treatment and a heat-sensitive resin layer formed on the substrate, and an image is formed by photolithography comprising the steps of exposing via a silver mask film and developing.
With the progress in computer image processing techniques and laser techniques, there has recently been proposed a computer-to-plate (CTP) system of digitizing image formation and irradiating with laser light based on the digitized image information to form an image directly on a photosensitive layer or a heat-sensitive layer of a lithographic printing plate, and thus an intense interest has been shown in the computer-to-plate system.
The lithographic printing plate used in the CTP system (this type of plate hereinafter abbreviated as a “CTP plate”) can be divided roughly into photosensitive CTP plates using a silver or a highly sensitive photopolymer photosensitive material, which is sensitive to visible light or ultraviolet light, and heat-sensitive CTP plate using a photosensitive material, which is sensitive to heat generated by absorbing near infrared light or infrared light, in the presence of an infrared absorber. In the case of the photosensitive CTP plate, a low power laser can be used because of its high sensitivity; however it requires an operation in a dark room and is insufficient in handlability and operability. On the other hand, although the heat-sensitive CTP plate has lower sensitivity than that of the photosensitive CTP plate, it has rapidly spread for the following reasons. That is, a small-sized high-power near infrared laser has recently been developed and the heat-sensitive CTP plate is excellent in operability in a lighted place such as lighted room because it is not sensitive to visible light and ultraviolet light and, furthermore, it has high resolution.
In particular, the development of the negative-working CTP plate has advanced because the area of the image area to be irradiated with a laser may be smaller than that in the case of the positive-working CTP plate. However, in a conventional negative-working CTP plate, the image area must be preheated after irradiation with a laser.
As the negative-working CTP plate which does not require the preheating process, there is proposed a heat-sensitive CTP plate comprising a substrate and a heat-sensitive layer containing water-soluble ammonium salt or amine salt, which is obtained by reacting a water-insoluble resin having a carboxyl group with ammonia or amine, and a photothermal conversion material, which absorbs light and converts light into heat, as an active component, formed on the substrate (see, for example, Japanese Unexamined Patent Application, First Publication No. Sho 58-162389). Regarding the heat-sensitive layer of the heat-sensitive CTP plate, the portion exposed to the laser is heated and, therefore, the ammonium salt or amine salt at the heated portion is decomposed and ammonia or amine is released and volatilized. As a result, the exposed portion is insolubilized. However, a press plate obtained by developing the heat-sensitive layer thus obtained had a problem in that satisfactory printing resistance cannot be obtained because the image area has low water resistance.
There is also proposed a heat-sensitive CTP plate comprising a substrate and a heat-sensitive layer containing fine particles made of a self water-dispersible thermoplastic resin and an infrared absorber (hereinafter abbreviated as an “IR absorber”) as an active component formed on the substrate (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 9-127683). In the case of the heat-sensitive layer of the heat-sensitive CTP plate, optical energy is converted into heat by the IR absorber in the portion exposed to laser light and fine thermoplastic resin particles are fused by heat to form a latent image. Since the solubility of the latent image made of fused fine thermoplastic resin particles in an alkaline developing solution is lowered, a press plate can be obtained only by washing out the heat-sensitive layer at the unexposed portion of the heat-sensitive CTP plate after exposure to a laser using an alkaline developing solution. However, the CTP plate has a problem in that fine particles at the exposed portion are completely fused with difficulty and cracking originating in the unfused portion occurs during printing and, therefore, satisfactory printing resistance cannot be obtained.
There is also proposed a heat-sensitive CTP plate comprising a substrate and a heat-sensitive layer containing a hydrophilic binder and fine hydrophobic resin particles dispersed in the hydrophilic binder as a main component formed on the substrate (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 9-171249 and Japanese Unexamined Patent Application, First Publication No. Hei 11-268225). However, the heat-sensitive CTP plate has a problem in that a press plate obtained by exposing it to a laser and developing is insufficient in water resistance of the image area and image defects occur during printing for a long time using dampening water and, therefore, satisfactory printing resistance cannot be obtained.